The invention concerns an imaging multi-slice nuclear magnetic resonance (NMR) method for recording N slices (N greater than 1) from a volume under investigation in a measuring object, wherein for complete image reconstruction, recording is carried out in m individual steps (m greater than 1), wherein the recording of each recording step of each slice takes the time TA.
A method of this type is known e.g. from DE 41 37 217 A1.
Imaging nuclear magnetic resonance tomography produces two-dimensional slice images of an object under investigation. To image a volume, sets of parallel slices are obtained which stretch the spatial direction in an orthogonal direction relative to these slices.
For recording an image, generally m partial steps are required in which n complex data points are recorded each. Image reconstruction from the m x n data points is carried out, depending on the spatial encoding, by means of a two-dimensional Fourier transformation (2DFT) [1] or by two-dimensional filtered reverse projection [2-3]. The magnetic field is varied in each case between two partial steps through temporally variable magnetic field gradients. In the 2DTF method, the variation is based on the switching of so-called phase encoding gradients. In this case, the partial steps are therefore also called phase encoding steps. Generally, a slice is excited several times for recording several phase encoding steps. The time between two excitations, the so-called repetition time TR, is in the magnitude of the NMR relaxation times and determines the image contrast.
Slices of a certain thickness can be selectively excited by means of frequency-selective (narrow band) RF pulses, which are irradiated under a constant magnetic field gradient. Multi-slice imaging defines a class of methods, which excite and read out successively several slices within a repetition time TR [4]. In each repetition, another phase encoding step of each of these slices is recorded (FIG. 1, left half of the illustration). The slices, which are excited in a repetition period, define a volume. This remains identical until complete recording of all slices of the volume and is excited in the same slice sequence in each repetition period.
The time interval between two successive excitations of the same slice equals the repetition time TR for all slices. All slices therefore have identical image properties. The volume is recorded in m repetitions within almost the same recording time required for recording one slice with equivalent image properties.
Modification of the conventional multi-slice method is known from the above-cited DE 41 37 217 A1. The slice positions of two phase encoding steps, which are principally associated with one, slice and recorded successively, differ by an increment, which is small compared to the slice thickness. Spatial distribution of the phase encoding steps permits reconstruction of additional overlapping slices without additional data recording through corresponding combinations of the phase encoding steps [5,6].
This method eliminates gaps between the reconstructed slices. In the conventional method, gaps are produced by the fact that the pulses used within a repetition time must have a certain separation from each other due to their mutual influence from the pulse slopes [7,8]. To obtain continuous or even overlapping cover of a volume, two or more mutually offset volumes must be recorded in sequential measurements.
The number of slices, which can be recorded in a conventional multi-slice measurement, is limited by the repetition time TR with given contrast. If this number is not sufficient to cover a desired volume, further slices must be recorded sequentially in subsequent measurements. This requires an integral multiple in measuring time. Further disadvantages for the examination can result if it is not possible to carry out e.g. a second measurement in the same respiratory rest position.
Larger volumes exceeding the homogeneity of the coil sensitivity or of the B field must also be recorded sequentially. The measuring object is displaced between the recordings. Continuous recording e.g. under continuous advance of the measuring object is currently not possible with a multi-slice technology.
In contrast thereto, it is the object of the present invention to improve a method of the above-described type such that the above-discussed disadvantages are eliminated. The invention is supposed to present in particular a new multi-slice method with the aim of making the number of slices recorded in one measurement independent of the repetition time which determines the image contrast.
In accordance with the invention, this object is achieved in an effective fashion in that the recording is carried out in r repetition cycles (r greater than m) and data from P slices (P less than N) is acquired during each of these repetition cycles,
wherein for recording the complete data set of m*N recording steps, the number of repetition cycles is r=m*N/P, wherein possible steps which remain due to the non-divisibility of m*N by P are attached or ignored,
wherein the spatial encoding during each repetition step can but need not necessarily vary from slice to slice as long as all m spatial encoding steps are recorded for each slice after r repetitions,
and wherein also the position of the P slices can be but need not necessarily be different within the investigation volume defined by the total number N of slices to be recorded as long as the data for reconstruction of N slices within the investigation volume is complete after termination of the recording.